In the course of examining gender differences on fly longevity, researchers here find sizable variations in life span between repeated studies. This variation is thought to derive from differences in maintaining a fly population, such as the dietary composition and season of the year. They suggest that this calls into question the detailed data obtained from much of the work involving aging and age-slowing interventions in flies. Reproducibility is critical to establishing whether or not observed effects are real. Flies may thus be a poor choice of model organism for any initial investigation of means to slow or reverse aging.
While our original goal was to understand how genetic variation played a role in costs of reproduction, we discovered strong cohort effects from one study to the next, especially across years. While we did not interrogate environmental and husbandry effects during the course of this experiment, we can surmise that these two factors were the major drivers of the differences we saw between the two replicate experiments, as genetic backgrounds were mostly comprised of iso-female lines.
The largest discrepancies between two years were seen with regards to maximum lifespans. While median lifespans were not changed to a large degree between years/seasons, maximum lifespans were significantly longer in summer 2019 in both sexes. We have several hypotheses, all related to fly husbandry, that could potentially explain this discrepancy. Our most likely, and anecdotally supported, hypothesis is that flies living in the summer are able to maintain better water homeostasis than those in the winter. Even though the incubators were set to approximately 60% humidity, we know that these often fluctuate. A second hypothesis for our observations is that the two experiments were done on slightly different media. As fly diet can have a huge impact on health and longevity, this could be contributing to our observed differences.
This lack of reproducibility in significant results between our two cohorts suggests that for certain questions the use of iso-female strains for determining genes that affect different phenotypes will require exquisite attention to husbandry details. The Drosophila Genetics Reference Panel (DGRP) has been used over the past decade to measure dozens of different biological phenotypes with conclusions about the genes playing a causal role in the phenotypes in question. However, if small environmental perturbations can make such differences in something a fundamental as sex differences in longevity, it is possible that many phenotypes may be more sensitive to subtle environmental variation than is generally supposed. As the fruit fly is used as the primary model organism to test novel compounds for their lifespan-extending effect, our results suggest that reproducibility between and even within laboratories might prove difficult.